Wire sealers are widely used for sealing plastic films in the packaging field. In such sealers, a heated wire is pressed against a pair of film layers and a back-up pad. The film layers are sealed to one another and the composite is cut by the heat and pressure of the wire. Typically, such wires operate at wire temperatures between about 450.degree. F. (232.2.degree. C.) and 700.degree. F. (371.1.degree. C.).
Such sealers may include a single sealing wire. However, most typically, such sealers include a pair of wires to form an L-seal.
A major problem with such wire sealers results from an inability to carefully control the temperature of the wire. Because of this, wire sealers are operated to reach a maximum temperature along a time-temperature response curve, at which time power is cut to the wire and the wire cools. Since accurate temperature control is impossible, these sealers must operate at maximum wire temperatures far above that necessary for film sealing. Plastics films may commonly be sealed at film temperatures between about 350.degree. F. (176.7.degree. C.) and 450.degree. F. (232.2.degree. C.).
When films are subjected to temperatures in excess of their sealing temperatures, the films produce smoke and resin build-up on the wire and the back-up pad. This build-up, along with the higher than necessary temperatures, result in quick degradation of the wire and back-up pad. The smoke is an environmentally objectionable occurance.
Thus, it is a major objective of the present invention to carefully control the wire temperature in a wire sealer. This results in an ability to seal at lower wire temperatures than previously possible, substantially reducing or eliminating smoke and build-up during the sealing operation.